How to invoke and interpret the Apple hardware tests

Regardless of the condition of your OS installation, if your system's hardware is not working properly then you will undoubtedly see undesired behavior that can stem from slowdowns and hangs to full system crashes and data corruption.

Because of the importance of having working hardware, Apple includes a hardware test routine on all of its new Mac systems, some of which are on the boot drive of your Mac, an others that are on the included OS X installation DVDs that came with older systems.

If you experience problems with your system crashing, hanging, or overheating with no change in the fan speed, random changes in fan speeds, or peripheral components not working, then one step that can help determine the problem is to run a hardware test.

The hardware tests are built for a specific system, so if you have a restore disc that contains tests for a Mac Pro, you will not be able to use this on a Mac Mini or iMac system. Additionally, tests for a newer model iMac may not work properly on an older iMac system, with the same restriction also applying for other Mac models.

Running the hardware tests
To run the hardware tests on your system, if your system came with a gray restore disc then insert it into the DVD slot and then start up or reboot your system, otherwise just skip this step. Then hold the D key down on the keyboard immediately after hearing the boot chimes, ensuring it is done before the gray startup screen appears. If your system did not ship with a restore DVD then the test software may need to be downloaded from the Internet, in which case the system will prompt you to join a network.

Apple includes the hardware tests on the hard drives of all modern Macs, but if you have formatted your drive then you will not have the tests on it anymore. In these situations you will need an active Internet connection to run the tests, but in some instances, router security routines may prevent a Wi-Fi connection from working when the system is in the diagnostics mode, so having an Ethernet cable to establish a hardware connection is a good option to have.

Apple's hardware test window will look like this. Click the big "Test" button to run all the hardware test routines.
Apple

When the tests load you will see a little symbol of a computer and a memory chip on the screen, with a progress bar under it. After the tests load you will be asked to choose the language to use for the hardware tests, followed by the main testing window appearing on the display. In this window you will see three tabs: one with information about the tests, another to run the tests, and a third to display a system report or hardware profile.

To run the tests, select the "Hardware Tests" section and click the big "Test" button in the box to the right of the window. The tests will check your CPU, GPU, memory, firmware, and other components, but if you wish to run an extensive memory test then you can do this as well by checking the check box under the Test button. Doing this will have the system meticulously test each section of RAM, but in doing so will increase the testing time from about 5 to 10 minutes to 30 to 60 minutes or longer, depending on the amount of RAM you have installed on your system and the speed of the RAM.

When the tests are run, you may hear the system's fans spin up at high speeds, but this is a normal occurrence for the system to when the hardware is being used without an operating system loaded.

The error codes
If the tests determine there is an error with your system, they will display an error code similar to the following:

4SNS/1/1/4000000 TL0P-130

The codes may be a bit cryptic, but you can contact Apple Support to see what they mean. As a general reference, here are some of the breakdowns for what the error codes mean, so if you see an error that begins with one of these codes, then it means the specified component listed at the end of the code is not working properly:

In the case of a sensor error (4SNS), to determine what type of sensor is being referenced, check its first letter, which should either be "I" for current, "T" for temperature, or "V" for voltage. Then check the second letter against the following list to see what component it refers to:

In the example mentioned above, a sensor called "TL0P-130" is not working properly, which is an LCD temperature sensor in a portable system.

What to do
When it comes to addressing the error code situation, unfortunately many times there's not much that you can do. In most cases if a hardware component is not working then you will need to get it replaced; however, there are some instances where you can try another step or two before having your system serviced.

If you get an error that deals with an Ethernet controller, the main logic board, a logic board controller, processors, or sensors, then try rebooting the system and resetting the system PRAM to see if the problem clears. If you are getting a problem with the fan speeds or any sensors then you can also try resetting the system management controller, but in addition to these resets do check the fans for physical obstructions (dust and dirt) or for mechanical issues.

If you see a memory or AirPort card error code, then your best bet would be to first try resetting them in their connections on the motherboard, but ultimately replace them if you cannot get the errors to go away on subsequent tests. If you are unable to perform these tasks yourself, then you should be able to have it done at an Apple Store.